Can you use a microwave to heat oils?

I’ve been asked about this on a number of occasions and having never really used a microwave to do this job myself I’ve always given a well thought out but personally un-tested response along the lines of:

‘well microwave ovens have been designed to take advantage of the vibration of water molecules. Oils contain very little water and therefore require a lot of energy to heat up this way. That makes it an inefficient method at best and at worst it is dangerous as the container tends to heat up quicker than the oil and can lead to burns’.

But today something different happened.

The question came up in a forum I have joined and the question wasn’t directed at me so I had no obligation to answer. However, I watched as half the people came back with a ‘yes of course, I always use the microwave’ and half with ‘no, microwaves are not good for that’. Some in the not good camp did furnish their answers with a little bit of ‘microwaves kill your DNA’ but that’s another story. What I had become fascinated with was the idea that everything I thought I knew about microwaves might indeed be wrong! With that on my mind I set about to find out some facts.

My original answer that microwaves heat water turns out to be correct. Microwaves work best at heating polar molecules and water is that, oils are generally not polar (they are non-ionic) and so don’t convert the electromagnetic radiation to heat as effectively as water does. This does indeed lead to them not heating up so efficiently.

But oils do heat up. I tried it and as much as I didn’t want them too (as if they didn’t heat it would prove me, the genius right) they got warm.

And that confused me.

So the oils MUST be able to convert the microwave radiation to heat somehow. What is going on?

I remembered a concept called ‘specific heat capacity’ from my uni days and decided to check that out.

Specific Heat Capacity is the amount of energy needed to raise the temperature of a substance.

It turns out that water has a pretty high Heat Capacity at 4.185 (Kj/Kg.K)

Glass has a heat capacity of 0.67 (Pyrex 0.753)

In practical terms this means that water takes over 6 times as much energy to heat it 1C than glass does!

What this means in our microwave scenario is that the glass container holding your microwave cup of water will get very, very hot much quicker than the water. This may not be great in our microwave example as because the waves are coming at the glass from all angles it will get hot all over and not just where the water is, this might make it very hard to handle. When we heat water in a glass on a hot plate the heat is coming from the bottom up so the top stays cooler for longer.

So what about oils?

I found a paper that said Olive oil has a Heat Capacity of 1.970 Kj/ Kg.K, Linseed Oil has a capacity of 1.84, g mixed vegetable oil 1.67, Sesame 1.63, Soybean 1.97.

So these oils take less energy to heat than water (they don’t store as much energy as heat) so it can’t be that there just isn’t enough energy produced in a microwave to heat them. Clearly if that was the case water wouldn’t heat.

AND it is true that microwaves vibrate water molecules which is how water eventually heats.

And we do know that while oils aren’t made of water they are not anhydrous – some vegetable oils and butters contain up to 15% moisture (fresh pressed Shea) although it is more usual for it them to contain less than 1%. Could THAT be heating the oil?

But then I had another idea…..

Is the heat transferring from the container to the oil?

Just like I said above the glass water container gets hot when you heat water in the microwave. It also gets hot when you try to heat oils but this time it gets VERY hot. I developed a theory that the microwave energy is heating the container and then the container is heating the non-polar oil. Indeed this radiant heat is a pretty normal cooking method so why couldn’t that work….

I tried putting oil on paper and it took ages to get warm – much longer than when I had it in a glass – I’ll do a full experiment in our lab once I get a new microwave as this would be good for my chemistry classes.

I then put oil on a piece of balsa wood – why? Because balsa wood has a specific heat capacity of 2.9 which means it takes nearly 4 1/2 times more energy to heat that. The oil took ages to heat – again I need to do this properly but you can see that it looks to be worth the time investment……

But does exploring this theory help us to answer the original question?

Well I think it does in part. Basically you can heat oils in the microwave but keep in mind that it is likely the heat is being transferred from your container to the oil and the oil is heating that way. With that in mind it is an inefficient and potentially dangerous way to heat an oil when compared to putting it in a double boiler (Baine Marie).

And is that all we should be concerned about?

Actually no, during my research I found a paper published in 2003 called ‘Changes occurring in vegetable oils composition due to microwave heating’ by the National Research Centre, Fats and Oils Department, Dokki, Cairo Egypt.

This paper found that oils heated in the microwave changed chemically. The tocopheryl content of the oils reduced, colour changed and peroxide values increased (indicating a degradation of the oils). The oils were microwaved for 18 minutes and sampled at 2 minute intervals. During this test temperature rose dramatically – at 2 minutes the temperature was at 120C and by 18 minutes the temperature was up to 227C. However, I believe that it is the temperature rather than the heating method that is to blame for this oil breakdown!

During cosmetic manufacture we are careful not to take our oil phase above 100C and more usually to 70-80C. This is a) because economically it makes no sense to heat things past their maximum melting points – heat is energy and energy costs money, b) most cosmetic factories heat stuff using water power and water boils at 100C unless it is under pressure – most cosmetic manufacturers don’t utilise high pressure and C) heating oils excessively changes their chemistry and facilitates their degradation.

So what does that mean for our ‘can you use a microwave to heat oils’ question?

Well, if a microwave can heat an oil to 120C in 2 minutes then it is too powerful for the job in hand and has the potential to break the oils down, taking away the natural antioxidants and ‘goodness’ that we are paying for. That seems a bit silly.

Further, it is likely that the container will heat up so much that it will be difficult to handle.

Finally microwaves are just another thing in the lab to keep clean.

For completions sake I am still wanting to do a few more experiments but I’m happy enough with what I have found out to conclude that I for one will not be heating my oils in the microwave anytime soon. I just don’t trust myself not too burn me, the or both of us.

Like this:

Related

Thanks for sharing this- I have some heating oils that I was wondering if I should put in the microwave. After reading this, I don’t think I will heat up the heating oils in the microwave, even if it does seem more convenient! Thanks again,

I was curious about this as well, so I simply poured a small amount of canola oil in a glass and put it in the microwave. After being in there for less than 20 seconds, it popped and there was a flash of light, so I stopped the “experiment” (don’t want to break my parents’ microwave). I’m confused because I know oils have a lack of polarity, and it wasn’t exceptionally hot when i took it out (I could still touch the oil with my bare finger). Since it wasn’t very hot, I don’t think it would have splattered onto the light bulb or anything to cause the light flash. I just did this out of curiosity and wasn’t expecting the experiment to be so eventful. Any thoughts?

It might have been the glass that was flashing. The oils would heat up slowly relative to the glass so in effect the microwave is heating the glass (which is full of inperfections and would probably heat unevenly and shatter) and then the glass transfers its heat to the oil. It might work better in a plastic container and be less dangerous.

I just want to thank u about that article . it was very useful to me , since i was heating my oils in the microwave for my hair oil mask , i saw that my hair fallen so badly making me very shocked about that .. I got a conclusion now that ” Microwave changes their chemistry ” … for sure i won’t ever and never again using microwave for heating my oils … thanks
dear

That’s sort of what I was getting at with this piece. You can heat oils in a microwave but they might be the last thing to get the heat- the container being the first – so really you are heating the oil through transfer of heat from the container not the microwaves 🙂

Radiant heat will heat the oils. The reason that microwaves are not a great way to heat oils though is because they take a lot of energy to raise the oils temp which is a waste and can also lead to the container cracking before the oil gets too hot. Heat isn’t necessarily a bad thing for oils (vegetable carrier oils). Some people feel that they have to keep their oils cold, especially cold-pressed oils, but this is not strictly true. While it is true that some trace ‘actives’ within the oil might start to break down when the oil is hot, usually that requires a significant amount of time (longer than the average cosmetic batch manufacture) and sustained heat of 80C or more. I say generally because there is always an exception, highly coloured oils might start to lose their colour and ‘change’ earlier than that. We often add our speciality oils at the cool down phase of an emulsion just to help ensure they don’t change but this may well be a conservative precaution rather than a necessity in many cases. The American Oil Chemists Association has some data on this but finding comparison data between oils is not usually that easy and no, I haven’t carried out any analytical testing on this myself at this stage.

Hi to all of you.
I am building a machine where I will heat gear oil up to about 600 degrees and my oil is in a 2-pipe circuit where a pump pumps the oil around.
can anyone tell me how to heat my oil whit microwave in this way.what design should I use.

Heating oil in a 2-pipe circuit is usually done by using either gas or electric power. What you are describing is a radiator and not a microwave. I don’t think a microwave with its vibrational energy would be the best source of power for your oils.

It’s all about electromagnetic parameters – in this case the critical one being dielectric loss. Just FYI, microwaves are not ‘tuned’ to a frequency that makes water molecules jiggle extra violently – that’s an old wives tale. That frequency is what we call the absorption band and their are several in the many GHz range – note this is different to the Lamar spin frequency used in MRI which are much lower in frequency. Loss tangent also relates inversely to skin depth ie how deep does the electromagnetic energy penetrate into an item before it decays away. The military have millimeter wave guns now which give people a nasty case of sunburn and makes them feel like they are burning so they turn tale and run – these guns are tuned to the water absorption bands. However, Water, unlike many others (except liquids like Methonol etc – don’t put that in your microwave), does have an especially high loss tangent which means that you can heat it effectively under RF radiated power at many frequencies ie 900MHz. The loss tangent does change with frequency though and in submarines we use very low KHz frequencies to transmit on close to the surface for this reason. They use pressure waves for everything else – sonar pinging. Now the problem with oils is that they have very low loss tangents – glass is usually higher meaning the glass jar gets hotter than the oil and then it transfers via convection. Similar is wax which is why in the old days they used to cover electronic components used in things like radio’s and transformers with it. It’s also the reason they use it in large power transformers – it convects the heat away from the windings of a power transformer but doesn’t heat up due to the electromagnetic field. Next time you cook anything in the microwave, notice how hot the silica glass plate gets. When microwaves are tested for efficiency they usually put a paper barrier or similar between the plate and the water load so that convection doesn’t screw the results. In essence its about how ‘Lossy’ your material is to electromagnetic waves coupled with the amount of power you pump in. Now as far as flashing occurs – the manufacturers tell you – or used to – not to run your microwave empty. This is because the lack of loss in the cavity means that the fields add in certain patterns – modes – and with no loss you get very high electric fields – in the 10’s of Kilovolts. If one of these high electric field points occurs close to the generator of power (magnetron) it has the potential to destroy it. So you can get flashing due to arcing (lightning) across points of high and low voltage.